This invention relates to improving the leach resistance of copper oxide and, in particular, to enchancing the leach resistance of the copper oxide coating on the copper circuits of printed boards.
Printed circuit boards, including multilayer printed circuit boards, are well-known and are used extensively in electrical equipment. Basically, a flat sheet or layer of dielectric material such as epoxy, polyester, or other resin, contains the desired circuit design imprinted on its surface in the form of copper conductor circuits.
Two of the most used methods to form the copper conductor circuits are known as the subtractive method and the additive procedure. In the subtractive method, for example, a laminate of dielectric coated with a thin copper foil about 1.4 mil or 2.8 mil thick is covered with a photosensitive resist and a latent image of the desired circuit is formed in the resist. The remaining portion of the copper foil, corresponding to the non-circuit areas of the final circuit layer, is left unprotected and is etched away in a suitable solvent. The resulting circuit layer is then treated in a solvent to strip the remaining resist coating on the retained copper foil. Regardless of the method used, however, the resulting printed circuit layer contains copper conductor circuits on its surface in the desired circuit design.
Multilayer printed circuit boards are desirable for many applications since they provide good packaging density an weight and space are conserved. Multilayer boards are, in general, built from a number of printed circuit layers which are laminated together with the circuits being electrically connected by through-holes in the board which are coated with a conductor such as copper.
One of the problems confronting the industry however, is the adhesion of the copper circuit to the dielectric material since, without good adhesion, delamination of the board and electrical malfunctions could occur. This problem has been extensively researched and one method to improve adhesion is to chemically treat the printed circuit layers before lamination with oxidants to form a copper oxide coating on the copper surface. An alkaline aqueous solution of a chlorite salt is typically used. It is hypothesized that the copper oxide coating has a roughened surface which promotes adhesion between the copper circuit and dielectric substrate.
U.S. Pat. No. 4,409,037 discloses a method for improving the copper oxide adhesion to printed circuit substrates by employing an aqueous solution of an alkali metal chlorite and sodium or potassium hydroxide at specific concentration levels. U.S. Pat. No. 4,512,818 also utilizes an alkaline chlorite solution but includes a minor amount of a water soluble or dispersible polymer in solution to improve the adhesion properties of the resultant copper oxide coating. Both patents extensively discuss the preparation of printed circuit boards and the compositions and processes used to form adhesive copper oxide coatings and, including the patents and references referred to therein, are all incorporated herein by reference.
The use of copper oxide coatings, including the recent improved copper oxide coatings, have not, however, solved all the adhesion-type problems in printed circuit board manufacturing processes. During the through-hole plating steps, for example, the copper oxide coating is attacked by processing solutions such as HCl based catalysts, and acidic copper electroplating baths and leaching of the oxide occurs resulting in a "pink ring" around the hole area. The pink ring area has a low copper to dielectric bond strength and, in general, the larger the leached area the lower the bond strength and integrity of the board.
Depending on the use for the board and/or board manufacturer, the goal is to have substantally no leaching of the copper oxide, i.e., no pink ring, and it is very important that pink ring be minimized in the manufacturing process to avoid increased cost due to board rejection.